There’s a phantom fat lurking in many of our favorite foods, including cookies, crackers, and the crust of Mom’s apple pie.

The secret of trans fats is getting out, however, thanks to the 2003 ruling by the Food and Drug Administration (FDA) that required all companies to list trans fats on their food products beginning in January 2006. Prior to this date, trans fats remained secret because they were not listed on food product labels and they did not fall into any defined fat group.

Although the chemistry behind the production of trans fats was first elucidated in 1907, they only became widespread in the 1960s in response to growing concerns about the artery-clogging effects of saturated fat. Food manufacturers found that including trans fats in their products allowed them not only to decrease the amount of saturated fat on the labels, but also to reduce cost and increase the shelf life of their products. Things were looking up for those who wanted to have a healthy diet; they could eat all the same great-tasting foods without the detrimental effects of saturated fats.

What food manufacturers had done was a sneaky trick. Saturated fats, or artery-clogging fats, are composed of a row of carbon atoms where every free space in the structure is occupied by a hydrogen atom. This organization allows the fats to pack tightly together, making them dense and more likely to aggregate and be obstructive. Unsaturated fats, the heart-healthy fats, look almost identical, only they are missing one or more hydrogen atoms. This subtle change in architecture affords less rigidity and makes them less likely to clog arteries.

Although saturated fat is considered relatively unhealthy and unsaturated fat is healthier, both of these structures exist in nature. Trans fats, on the other hand, are mostly synthetic. A trans fat is made by adding heat to an unsaturated fat, followed by the laboratory addition of hydrogen atoms. This chemical procedure creates a product in which the hydrogen atoms reside in areas of the molecule where they are not normally tolerated, and the structure becomes permanently altered. The products are considered unsaturated by pure chemical definition, but the transformation has been so severe that trans fats cannot really be grouped into any previously defined fat category.
The result is that consumers incorporate trans fats into their many cells and cellular processes. Doing so leads to an offset in balance, much as if you were to replace the batteries in your TV remote with AAA instead of AA; the battery still has energy, but the system won’t work. This realization prompted a battery, if you will, of medical studies, including an in-depth study that followed the intake of trans fatty acids in 85,000 women for eight years. This study found 431 cases of newly developed coronary artery disease that correlated with an increase in consumption of trans fatty acids by these women. In 2002 the National Academy of Sciences recommended that dietary intake of trans fats be limited.

Following the initial alarm, results from clinical trials suggested that saturated fat and trans fat probably contribute equally to heart disease. They do so, however, in different ways. Saturated fat is an equal-opportunity cholesterol booster. Both the good cholesterol, known as high-density lipoprotein (HDL) and the bad, low-density lipoprotein (LDL) levels are increased in response to saturated fat consumption. Having too much LDL in the body leads to cholesterol deposits on artery walls, which is only mildly compensated for by the subsequent increase in HDL. Trans fat practices more discrimination by raising LDL and reducing HDL. The arteries in this case have no reprieve from HDL, and therefore are more susceptible to blockage.

Although saturated fats occur naturally in products like beef fat and whole milk, making them very challenging to avoid completely, trans fats are engineered by manufacturers and could be removed on behalf of consumers. Yet they are ubiquitous, having been used in food products since 1960.

“We simply don’t have a supply line yet that provides the oils that don’t have [trans fats],” says Laura Kettel-Kahn, an obesity researcher and adjunct assistant professor at Rollins School of Public Health.

Until such oil is available, healthy, good-tasting products may be scarce. The heating process that creates trans fat oils somehow makes food more delectable, so the challenge is analogous to making s’mores without a fire. Sure, it will still taste pretty good—you’ve got your chocolate, your marshmallow, and your graham cracker—but unfortunately, it won’t melt in your mouth.”